Capacitance multiplication network

ABSTRACT

A steering network employing first and second clock switching transistors having their emitters connected in common to a clock pulse source with first and second capacitors each connected between a different base electrode and a point of reference potential is described. In addition, clock signal responsive means provides charge paths for the capacitors under the selective control of input signal responsive means.

United States Patent William C. Blumenste'in Cinnaminson, N.J. 470,869

July 9, 1965 Nov. 30, 1971 RCA Corporation Inventor Appl. No Filed Patented Assignee CAPACITANCE MULTIPLICATION NETWORK 10 Claims, 6 Drawing Figs.

US. Cl 307/291, 307/246, 307/247 Int. Cl "03k 3/12, H03k 3/26 Field of Search 328/206,

[56] References Cited UNITED STATES PATENTS 2,885,574 5/1959 Roesch, Jr. 307/885 3,069,565 12/1962 Van Ness 307/885 3,259,757 7/1966 Lavin 307/885 OTHER REFERENCES Direct-Coupled Type Ring Counter," by R. W. Carney, pg. 7 32. MlL-l-lDBK-2l5, June 15,1960

Primary ExaminerJohn Zazworsky Allorney.lohn V. Regan ABSTRACT: A steering network employing first and second clock switching transistors having their emitters connected in common to a clock pulse source with first and second capacitors each connected between a different base electrode and a point of reference potential is described. in addition, clock signal responsive means provides charge paths for the capacitors under the selective control of input signal responsive means.

PATENTEnunv 30 I97! 3,824,425

SHEET 2 [1F 2 Z64 INVENTOR. Z259 Mam/d fizunfwiml BY WW CAPACITANCE MULTIPLICATION NETWORK This invention relates to electrical circuits and, in particular, to active steering networks.

Steering networks are used in conjunction with flip-flops (bistable multivibrators) to steer applied input signals to one or the other of the flip-flop input terminals, as in triggerable flip-flop, shift register and counter applications. Most known steering networks have one or more characteristics which are undesirable in integrated circuits. For example, they require capacitors of relatively large value. In a monolithic circuit, the required surface area is a direct function of the capacitance, while circuit yield varies inversely with the surface area. Thus, it is desired that the capacitors have low values. Further, the capacitors in the conventional steering networks usually are floating" in that neither plate of a capacitor is grounded. In an integrated circuit an advantage obtains when one plate of a capacitor is grounded; a diffused or junction capacitor can then be used. This eliminates problems encountered with metaI-oxide-type capacitors, such as pinholes and low-voltage breakdown. A further requirement of an integrated circuit is that the power dissipation be very low.

It is one object of this invention to provide a novel steering network whose operating characteristics are especially compatible with integrated-circuit requirements.

It is another object of this invention to provide an improved steering network which has time constant multiplication, whereby the capacitors may have small capacitance values and be small in physical size.

It is still another object of this invention to provide an improved steering network in which each of the capacitors has one plate connected to a point of fixed potential.

It is a further object of this invention to provide an improved steering network which has low-power dissipation.

Still another object of this invention is to provide an improved steering network which avoids a race condition when the network is used with a flip-flop in triggerable flip-flop, shift register and counter applications.

In the preferred form of the invention, first and second transistors have their emitters connected together and by way of a common emitter resistor to a control input terminal. The separate outputs of the network are derived at the collectors of these transistors. A first capacitor is connected between the base of the first transistor and a point-of-reference potential, and a second capacitor is connected between the base of the second transistor and a point-of-reference potential. The first and second capacitors are chargeable through third and fourth transistors, respectively, depending upon input signal conditions, when the voltage at the control input terminal has a first value. A charged capacitor is discharged by way of the respective first or second transistor when the voltage at the control input terminal has a second value, and current flows through that transistor to the output during the discharge period.

In the accompanying drawing, like reference characters denote like components, and:

FIG. 1 is a schematic diagram of a flip-flop and a steering network embodying the invention;

FIG. 2 is a block diagram indicating the connections to the steering network for a triggerable flip-flop application.

FIG. 3 is a block diagram illustrating the manner of connecting several flipflops and steering networks for operation as a counter;

FIG. 4 is a schematic diagram illustrating a modification of the steering network;

FIG. 5 is a block diagram illustrating the manner of connecting several flipflops and steering networks for operation as a shift register; and

FIG. 6 is a schematic diagram of another active steering network embodying the invention.

The active steering network of this invention is not limited in its application to any one particular type of flip-flop. However, for purposes of illustration and completeness, the FIG. 1 circuit includes a schematic diagram of a flip-flop with which the active steering network may be employed. The flip-flop is located above the dashed line 10 and comprises a pair of gates A, B which are cross coupled in the usual manner.

Gate A includes a pair of input diodes 12a, 140 having their anodes connected together at an input junction 15a at the base 16a of an NPN-transistor 18a. A resistor 20a is connected between the base 16a and the collector 22a, and a supply resistor 24a is connected between collector 22a and a source of suitable operating potential. designated +V,. Emitter 26a is directly connected to the base 30a of another NPN-transistor 32a, and is connected by way of a resistor 34a to the emitter 36a of the latter transistor and the base 40a of still another NPN-transistor 42a. A resistor 44a is connected between circuit ground and a point common to the base 40a and the emitter 36a. The collector 46a of transistor 32a is connected to the base 48a of an NPN-transistor 50a, and is returned to the +V source by way of a supply resistor 52a. Transistor 42a has its collector 60a connected by way of a resistor 62a to the emitter 64a of the transistor 50a, whereby the collector-emitter paths of these transistors are connected in series between circuit ground and the +V volt source. A first output, designated (1), is derived at a terminal 70a which is connected at the collector 60a.

Gate B is identical to gate A, and like components are designated by like reference numerals followed by the letter b."F or operation as a flip-flop, the (1) output terminal 700 is connected to the cathode of the input diode 14b in gate B, and the (0) output terminal 70b of gate B is connected to the cathode of the input diode 14a in gate A. In the operation of the flip-flop, the flip-flop may be switched to the SET state by applying an input signal of suitable value at the input terminal 72a to forward bias diode 12a and to render transistor 18a nonconducting. The voltage at the emitter 26a thereof then has a value close to ground potential, whereby transistors 32a and 42a are rendered nonconducting. The voltage at collector 46a of transistor 32a then rises in a positive direction to render transistor 50a conducting, whereby the (1) output voltage at terminal 70a has its most positive value. This voltage, when fed back to the cathode of input diode 14a, reverse biases the diode. Transistor 18b, in gate B, then is rendered conducting by base current supplied through the base bias resistor 20b. Resistor 20b, located between the base and collector of transistor 18b, prevents saturation of this transistor. With transistor 18b conducting, the voltage at its emitter 26b has a positive value, whereby transistors 32b and 42b are rendered conductive. Sufficient current flows through transistor 32b and its collector resistor 52b to lower the voltage at the base 48b of transistor 50b and thereby decrease conduction in that transistor. With transistor 42b in a conducting state, the (0) output voltage at terminal 70b is close to ground potential. This voltage, when fed back to the cathode of input diode 14a, maintains transistor 18a in a cutoff condition.

The flip-flop may be switched to the RESET state, by external means, by lowering the voltage at input terminal 72b to a value sufficient to cut off the transistor 18b. Alternatively, the flip-flop may be switched from the SET to the RESET state by diverting sufficient current from the base 16b to turn off the transistor 18b. That is to say, if a sufficient amount of the current normally supplied by way of resistor 20b is diverted away from the base 16b, transistor 18b will turn off, the (0) output at terminal 70b will become more positive, and this output will reverse bias input diode 14a and allow transistor 18a in gate A to turn on.

The active steering network embodying the invention is illustrated in FIG. 1 below the dashed line 10. This network includes first and second NPN-transistors a, 10% having their respective emitters 102a, 102b connected directly in common and by way of a common emitter resistor 103 to a first input terminal 105. The collectors, 104a and 104b are connected, respectively, to the flip-flop input points 15b, 15a at the base electrodes 16b, 16a of the transistors 18b and 18a. Essentially, the resistors 20b and 20a in the base circuits of the latter transistors serve as the collector supply resistors for the transistors 100a and 100b, respectively. Third and fourth NPN-transistors 110a and 11Gb have their emitters 112a and l12b respectively connected at the base electrodes 106a and 106b of the transistors 100a and 1041b. Collector 1140 is connected by way of a resistor 1160 to the control input terminal 105, and the collector 11417 of transistor 11Gb is connected to that input terminal by way of a resistor ll6b. An input terminal 122a, designated S,," is coupled by way of a resistor 124a to the base 126a of third transistor 110a. Another input terminal 122b, designated R,." is coupled by way of a resistor 1241) to the base 126!) of fourth transistor 1 10b.

A first capacitor 120:: has one of its plates directly connected to a point-of-reference potential, indicated by the conventional symbol for circuit ground. The other plate thereof is connected at a point common to the base 1060 of first transistor 100a. Base 1060 also is coupled to the input terminal i22a by way of a diode. This diode is illustrated as a transistor 1300 having its collector 132a and base 134a tied together and to the base 106a, and having its emitter 1360 connected to the input terminal 122a. It will be noted that the easy current flow direction across the base l34a-emitter 136a junction is opposed to the easy current flow direction across the base 106a-emitter 102a junction of first transistor 100a. In a similar manner, a second capacitor 12Gb is connected between the base 10Gb of second transistor 1011b and circuit ground. A transistor l30b is connected as a diode between input terminal l22b and the base l06b of transistor 10Gb.

First and second other diodes 140a and 140!) are connected, respectively, between the output terminal 70a and 70b of the flip-flop and the bases 126a and l26b of the third and fourth transistors 110a and ll0b. The diodes 130a, 130b, 140a and 140b are operative when the steering network is employed in a shift register application, as will be discussed more fully hereinafter.

FIG. 2 illustrates the manner in which connections are made in the FIG. 1 circuit for a triggerable flip-flop application. As illustrated in FIG. 2, the (1) output terminal of the flip-flop is connected to the input terminal designated 8,, and the (0) output terminal is connected to the input terminal designated R f Therefore, the FIG. 1 arrangement is operative as a triggerable flip-flop by connecting the output terminals 70:: and 70b of the flip-flop to the input terminals 122a and 122b, respectively, of the steering network. In order to more fully understand the operation of the circuit arrangement, let it be assumed that the parameters of the flip-flop are such that the (1) output voltage at output terminal 700 of the flip-flop has a value +3.3 volts when the flip-flop is in the SET state (transistor 18:: nonconducting), and has a value of approximately ground potential when the flip-flop is in the RESET state. The voltages at the (0) output terminal 70b are the complements thereof, that is to say, ground potential when the flip-flop is set and +3.3 volts when the flip-flop is reset. Let it be assumed further that the control signal 144 applied at the control input terminal 105 of the steering network has a value of either +3.3 volts or ground potential, corresponding with the output voltages of the flip-flop.

When the flip-flop is in the SET state, the (1) output voltage thereof is +3.3 volts. As indicated in FIG. 2, this voltage is applied at the S input terminal 122a. Ordinarily, the control voltage at control input terminal 105 has a value of+3.3 volts. With the, voltage values indicated, it may be seen that third transistor 1100 in the steering network is rendered conductive, whereby capacitor 120a charges in the polarity direction indicated to a value which is slightly less than +3.3 volts. Due to current flow through the collector resistor 116a, third transistor 110a saturates during a portion of the charge period. Since the collector saturation current is determined by resistor 1160, this resistor has a direct effect on the charge time constant for the capacitor 120a. A faster charge time could be provided by eliminating the resistor 116a. However, the resistor 116a (and the resistor 11Gb) provides immunity to noise spikes at the input terminal 105.

The (0) output voltage of the flip-flop is at ground potential at this time. With ground potential applied at the R input terminal 122b, transistor [10b is biased off, whereby second capacitor 12Gb does not charge through the transistor. In some integrated circuits, the capacitance between the collector and emitter terminals of a transistor is quite high. If this condition exists for the fourth transistor 110b, it would ordinarily be possible for second capacitor l20b to become charged through the collector-emitter capacitance of the transistor [10b even though the transistor is biased off. However, this result is avoided by the diode 1301). Since the voltage at input terminal 122b is close to ground potential, diode 13Gb will conduct if the voltage at the ungrounded plate of second capacitor 1201) becomes more positive than about 0.7 volt, and will limit the voltage across the capacitor to about that value.

When the control voltage at input terminal is lowered to ground potential, transistor 100a becomes conducting by virtue of the positive charge on the capacitor 120a. A current then flows, in the conventional sense, through the resistor 20b in gate B, the collector l04a-emitter 102a path of transistor 100a and common emitter resistor 103. Current is thereby diverted from the base 16b of transistor 18b and causes this transistor to turn off. Thereupon, transistors 32b and 42b also turn ofi and transistor 50b (gate B) turns on, raising the (0) output voltage at terminal 70b from ground potential to +3.3 volts. This output voltage, when applied over the crosscoupling connection, reverse biases input diode 14a, whereupon transistor 18a turns on and the (1) output voltage falls from +3.3 volts to ground potential. Thus, the state of the flipflop is switched when the control input signal 144 is applied.

Capacitor 120a discharges when the transistor 1000 is rendered conductive by the negative-going control pulse 144. This capacitor is discharged by the base current in transistor 100a, and transistor 1000 becomes nonconducting when the voltage across the capacitor is reduced to a sufficiently low value. The width or duration of the signal applied to the flipflop is determined by the time it takes to discharge capacitor 120a. In turn, this period is a function of the value of the capacitance, the value of common emitter-resistor 103, and the beta of transistor 100a. Essentially, the transistor 100a multiplies the time constant by a factor equal to the beta of the transistor because only a fraction of the emitter 1020 current flows in the base 106a circuit to discharge the capacitor. It is this fact which allows the use of a capacitor 1200 of small value, while still providing a sufiiciently long time constant to assure full switching of the flip-flop. The transistor 1000 can be considered as being a capacitance multiplier.

Resistor 103 causes transistors 100a and 10012 to act as a differential switch increasing the back bias cutoff voltage on the base-emitter junction of the off transistor when the on half conducts, ensuring correct coupling action at transistors 100a and 10% to the flip-flop.

The third transistor 110a is essentially disconnected from capacitor a when the control voltage 144 is at ground potential. This result is achieved by virtue of the fact that the collector l14a-base 126a junction becomes forward biased. The voltage at the base 126a is determined by the values of the base resistor 124a, collector resistor 116a, and the voltage at input terminal 122a. By suitable choice of resistor ratio, the base voltage I26a may be about +1 volt when the input voltage at terminal 122a is +3.3 volts. With this low base 126a voltage, the capacitor 120a cannot recharge while the trigger pulse 144 is present. In like manner, fourth transistor 110!) is maintained nonconducting even after the flip-flop switches and the voltage at input terminal l22b is raised to +3.3 volts. Therefore, capacitor 12Gb cannot charge. Thus, both of the transistors 100a and 10Gb are maintained in a nonconducting condition after capacitor 120a has discharged, whereby there can be no further triggering of the flip-flop and no race condition can result.

At the termination of the triggering pulse 144, the voltage at control input terminal 105 rises to +3.3 volts. Fourth transistor 11% then is rendered conductive and capacitor 120b charges through the fourth transistor. The other capacitor 1200 does not charge at this time because the input voltage at terminal 1220 is now at ground potential. When the next trigger pulse 144 is applied, second transistor conducts until the capacitor 12% has discharged through the base 106b-emitter 102b path. During the time that transistor 10% conducts, a current flows through the base bias resistor a gate A). The value of this current is sufficient, at least during the initial surge, to turn off transistor 18a, whereby transistors 32a and 42a are rendered nonconducting and transistor 50a is rendered conducting. The (1) output voltage at terminal 70a then rises from ground potential to +3.3 volts and, when applied at the cathode of input diode 141), back biases this diode and allows transistor 18b to conduct. The (0) output voltage at terminal 70b then falls from +3.3 volts to ground potential and maintains transistor 18a in a nonconducting condition. The flip-flop then has been triggered from the RESET to the SET state.

As discussed above in connection with the triggering from the SET to the RESET state, transistors 100a and 11% are inoperative to charge the capacitors 120a and 120b, respectively, while the trigger pulse 144 is present. Therefore, neither of the transistors 100a or 100b can conduct after the capacitor l20b is discharged, and there can be no further triggering of the flip-flop, whereby a race condition is avoided.

For a multistage binary counter application several circuits of the type illustrated in FIG. 1 may be connected in the manner illustrated in block form in FIG. 3. Each individual stage is connected in the same manner as illustrated in FIG. 2 and described hereinabove. In addition, the (1) output terminal of each stage is connected at the trigger input terminal of the next succeeding stage. Thus, the trigger pulse 144 of FIG. 1 is the (1) output of the next preceding stage. Since this voltage falls from +3.3 volts to ground potential when the preceding stage switches from the SET to the RESET state, it can be seen that a stage is triggered only when the preceding stage becomes reset, which is the proper operation for a binary counter.

Several circuits of the type illustrated in FIG. 1 may be connected for operation as a shift register in the manner illustrated in FIG. 5. As there illustrated, the (1) output terminal of each stage is connected at the R input terminal of the next succeeding stage, and the (0) output terminal of each stage is applied at the S input terminal ofthe next succeeding stage. A common shift pulse is applied at the ADVANCE (A) input terminal of each stage. This terminal is the control input terminal 105 ofFlG. 1.

The diodes 140a and 14017 (FIG. 1) which are connected between the (1) and (0) output terminals, respectively, and the bases of the transistors 110a and 110b are of importance in the shift register application for the following reason. When the flip-flop is in the RESET state, the voltage at output terminal 70a is at ground potential and the voltage at output terminal 70b is at +3.3 volts. This means that the voltage at the flip-flop input point 15b is at approximately +0.8 volt, the drop across input diode 14b. If the preceding stage also is in the RESET state, the voltage applied at input terminal 122a of the steering network has a value of +3.3 volts and the voltage at input terminal l22b is at ground potential. If it were not for the diode 140a circuit, the third transistor 110a would be rendered conductive at this time and capacitor 120a would charge in the polarity direction indicated. This charge would have no effect on the flip-flop when the next shift pulse 144 was applied since transistor 1000 would be rendered conductive and divert current away from the base of transistor 18b. However, transistor 18b is already nonconducting, whereby this current would have no effect.

However, it will be noted that input diode 14b is forward biased at this time. If capacitor 120a were permitted to charge during the nonshift period, the voltage at base electrode 106a of transistor 100a would become several volts positive relative to ground. The base 106a-collector 104a junction of the transistor would be forward biased, and a large forward voltage would appear across the series combination of input diode 14b and the collector-base junction of transistor a. This large forward bias may be sufficient to damage the transistor. This condition and result are prevented by the diode 140a. With ground potential at the (1) output terminal 70a and +3.3 volts at the input terminal 122a, diode 140a is forward biased and clamps the voltage at base 126a at approximately +0.8 volt. This value of base voltage is insufficient to render third transistor a conducting, whereby the capacitor 1200 cannot charge and the base 106a-collector 104a junction of first transistor 100a cannot become forward biased. In like manner, the other diode 140b serves to protect the second transistor 100b when the voltage at the (0) output terminal 70b is at ground potential and the voltage at the R, input terminal l22b is at +3.3 volts.

The transistors 130a and 130b, which are connected as diodes, also are of importance in a shift register application. Consider the conditions which obtain when the voltage at input terminal 122a is +3.3 volts and the voltage at input terminal l22b is at ground potential. Capacitor a then charges during the nonshift period. If the information in the shift register is changed by external means, as by applying signals at the input terminals S or R (FIG. 5), the voltage at input terminal 122a may switch from +3.3 volts to ground potential. In the absence of the transistor a, capacitor 120a could not discharge, whereby false triggering would result when the next shift pulse was applied at the control input terminal 105. Transistor 130a prevents this condition by providing a discharge path for the capacitor 120a when the input voltage at terminal 122a is changed from +3.3 volts to ground potential during a nonshift period. In like manner, the other transistor 13012 provides a discharge path for capacitor l20b when the input voltage at terminal 122b is changed from +3.3 volts to ground potential.

The active steering network as thus described in detail hereinabove, has the advantage that there is no power dissipation therein during the steady state condition. Also, the capacitors 120a and 12% may have a small value of capacity since the transistors 100a and 10012 act as capacitance multipliers. For the same reason, the common emitter resistor 103 may have a relatively low value. This feature is of importance in integrated circuitry because the area required for a capacitor or a resistor is a function of the value of capacity or resistance, and large area circuits result in low circuit yields. Furthermore, the fact that each of the capacitors 120a and 12012 has one terminal grounded permits the use of diffused capacitors in an integrated circuit. In the actual circuit, the value of common emitter resistor 103 is selected to provide a discharge time constant of sufficient duration to assure complete triggering of the flip-flop. The values of the collector resistors 116a and l16b may be tailored to provide a sufficiently fast charge time for the capacitors 120a and 120b, respectively.

By way of example only, the component values of the steering network may be as follows:

Capacitors 120a, 12011: 20 pf.

Resistor 103: 2 kilohms Resistors 116a, 1161;: 3 kilohms Resistors 1240, 124b: l0 kilohms In addition to its use as an active steering network for flipflops in the applications mentioned, a circuit embodying the invention also may be used to provide selective setting and resetting of the flip-flop under the control of clock pulses or gating pulses. This is accomplished by modifying the steering network in the manner illustrated in FIG. 4. The modification consists of providing separate emitter resistors 103a and l03b for the first and second transistors 100a, 100b, respectively. The lower end of each resistor is connected to a separate input terminal 105a or 105b, and separate clock sources 108a and 108b are connected between the respective input terminals 105a, 105b and circuit ground. In this arrangement, transistor 100a is rendered conductive by a clock signal from source 108a when its capacitor 120a is charged, and transistor 1001) is rendered conductive by its clock source 108b only when its associated capacitor 12012 is charged. By connecting the input terminals 105a and lb together, and employing a single clock source 108a or 108b, the network operates in the same manner as the FIG. 1 circuit.

In describing the operation of the FIG. 1 circuit, it was mentioned that first capacitor 120a charges, in the polarity direction indicated, when the voltage at the base of third transistor 1100 is positive (+3.3 volts) and the control input voltage at terminal 105 is positive. It was mentioned further that capacitor 120a is discharged through the emitter-base junction of first transistor 1000 when a negative-going control pulse 144 is applied. Transistor 100a then conducts during the discharge period to switch the state of the flip-fiop. This description of the capacitor discharge assumes that third transistor 110a (and fourth transistor l10b) has a low inverse beta, whereby capacitor 120 is not discharged by inverse emitter current in the third transistor 110a. A normal transistor usually has a sufficiently low inverse beta to prevent such discharge of the capacitor. However, the inverse beta of transistors manufactured in monolithic form according to some processes may be too high to achieve satisfactory operation of the steering network of FIG. I. The aforementioned problem is avoided in the embodiment of the invention illustrated in FIG. 6.

In FIG. 6, the flip-flop is illustrated in block form, and only those elements of the flip-flop necessary to an understanding of the invention are illustrated in the box. The active steering network comprises first and second NPN-transistors 202a, 20% having their emitter electrodes 200a, 2021) connected in common and by way of a common emitter resistor 203 to the control input terminal 205. The collector 204a of first transistor 200a is connected to the input point a of the flipi'lop, and the collector 204!) is connected to the input point 15b. It should be noted that these connections are reversed from the connections illustrated in FIG. I. This is due to a phase inversion which will become more fully apparent as the discussion proceeds. Third and fourth transistors 210a and 21Gb have their respective bases 212a, 2I2b connected directly to the control input terminal 205 and have their collector electrodes 214a, 2141: connected to the supply source of +V volts. The emitter 2160 of the third transistor is connected by way of a resistor 218a to the base of first transistor 200a. In a similar manner, the emitter 2l6b is connected by way of a resistor 21812 to the base of second transistor 20011.

A first capacitor 220a is connected between the base 206a of first transistor 200a and circuit ground. A fifth transistor 2240 has its collector-emitter path in parallel with the capacitor 220a, and has its base 226a returned to circuit ground by way of a resistor 228a and coupled by way of a resistor 230a to an input node 232a. The input node 232a is the output of an emitter follower gate comprising a pair of transistors 240a and 242a connected in the common collector configuration and having their emitters 244a and 246a connected together and to the input node 232a. The S input terminal 250a is connected to the base 248a of transistor 242a, and the (1) output terminal 700 of the flip-flop is connected to the base 252a of transistor 2400.

The base input circuitry to second transistor is similar to that of the first transistor 200a, and like components are denoted by like reference numerals followed by the letter b." The base 2481) of transistor 24212 is connected to the R input terminal 250b, and the base 252b of transistor 24Gb is connected to the (0) output terminal 701) of the flip-flop.

The operation of the FIG. 6 circuit will be described for a triggerable flip-flop application. In such an application, no inputs are applied at the S and R input terminals 250a, 2S0b. Let it be assumed that the flip-flop is in the SET state, whereby the (1) output voltage at the flip-flop terminal 70a has a value of +3.3 volts. This voltage is sufficiently positive to forward bias the emitter-base junction of transistor 240a, whereby the voltage at input node 2320 is about +2.5 volts. Fifth transistor 224a then is biased into saturation and provides a very low-impedance path across the terminals of the first capacitor 220a.

When the control input voltage 244 is at its high level (about +3.3 volts) third transistor 2100 is biased into conduction, and current flows, in the conventional sense, from the +V, source through transistor 210a, resistor 218a and transistor 2240 to circuit ground. First capacitor 220a cannot charge because of the low-impedance path thereacross provided by fifth transistor 224a.

At the same time, the (0) output voltage of the flip-flop is at ground potential, whereby input transistor 24Gb is nonconducting, and no current flows into the input node 23211. Accordingly, transistor 2241a also is nonconducting, and there is no low-impedance path across second capacitor 2201). Fourth transistor 210b conducts when the control input voltage is at the high level, and capacitor 220!) charges, in the polarity direction indicated, through resistor 2l8b and the collector emitter path of fourth transistor 2l0b. The resistor 2181) determines, in part, the charge time for the capacitor 22012 and is included in the circuit to provide noise immunity and prevent charging of the capacitor in response to noise spikes at the control input terminal 205. a

When the control input voltage 244 is switched from +3.3 volts to ground potential, transistor 210b turns off and provides a high-impedance path between its collector and emitter. Second transistor 200a turns on because of the positive charge on the capacitor 220b, and current flows through the transistor to the input point 15b of the flip-flop, switching the flip-flop from the SET state to the RESET state. Capacitor 220b discharges through the base 206b-emitter 202b junction of the transistor and the common emitter resistor 203. As in the case of the FIG. 1 circuit, the transistor 200!) remains conducting until the capacitor 220b has discharged, and the discharge time is determined by the value of the capacitor 22012, the resistor 203 and the beta of the transistor. Essentially, the transistor 200b provides capacitance multiplication.

Because the third and fourth transistors 210a and 2101; are biased in the off condition when the control input voltage is low, neither of the capacitors 220a, 2201: can recharge during the trigger portion of the cycle. Hence, there is no possibility of a race condition and double triggering. When the control voltage again rises to +3.3 volts, transistors 210a and 210!) are again biased into conduction. The (1) output of the flip-flop is now low, whereby transistors 240a and 2240 are nonconducting. First capacitor 220a then charges through resistor 218a and transistor 210a. Since the (0) output of the flip-flop is high, transistors 24017 and 22417 conduct. Transistor 2241) operates in saturation and presents a very low-impedance path across second capacitor 220b, whereby the capacitor cannot charge. Accordingly, when the next trigger pulse 244 is applied at the control input terminal 205, first transistor 200a conducts to discharge first capacitor 220a and to supply current at the input point 15a of the flip-flop, causing the flip-flop to switch from the RESET to the SET state.

In some cases, there may be a ringing on the trigger input line, and the voltage at the control input terminal 205 may become negative relative to ground potential. This condition is to be avoided since otherwise both of the transistors 200a and 20% may conduct and produce false triggering of the flipflop. This condition is prevented by connecting a diode 260 between the control input terminal 205 and circuit ground, the diode being poled so as to become conducting when the voltage at the control input terminal 205 tends to fall negative relative to ground. The diode 260 may be a transistor of PNP- conductivity having its base 262 connected at the control input terminal 205 and having its collector 264 and emitter 266 connected together and to circuit ground.

The active steering network of FIG. 6 may also be used with the flip-flop in a multistage counter application, in which case the control input terminal 205 is connected at the (1) output terminal of the preceding stage. The network also has application for use in a shift register, in which case connections are made to the S and R input terminals 250a and 250!) in the manner illustrated in FIG. 5.

What is claimed is:

l. The combination comprising:

first and second transistors of one conductivity type each having an input electrode, an output electrode and a common electrode;

means for connecting first and second output loads at the output electrodes of the first and second transistors, respectively;

an input terminal;

a relatively constant impedance means connected in common at one end to each one of said common electrodes and at the other end to said input terminal for increasing the impedance between the input electrodes of said transistors and said input terminal;

first and second capacitors each being connected between the input electrode of a different one of said first and second transistors and a point-of-reference potential;

third and fourth transistors connected to the input electrode of the first and second transistors, respectively, for selectively charging said first and second capacitors, respectively, to a first charge condition; and

means for applying a control signal at said input terminal,

said control signal having a magnitude to render conductive the first or second transistor when its associated capacitor is charged to said first charge condition.

2. The combination as claimed in claim 1 wherein said rela tively constant impedance means is a resistor.

3. The combination comprising:

first and second transistors each having an input electrode,

an output electrode and a common electrode;

means for connecting first and second output loads at the output electrodes of the first and second transistors, respectively;

first and second input tenninals;

a first resistor connected in circuit between the common electrode of the first transistor and the first input terminal;

a second resistor connected in circuit between the common electrode of the second transistor and the second input terminal;

a first capacitor connected between the input electrode of the first transistor and a point-of-reference potential;

a second capacitor connected between the input electrode of the second transistor and said point-of-reference potential;

first and second signal controlled input means connected to the input electrodes of the first and second transistors, respectively, to control the charge on the first and second capacitors, respectively; and

means for applying control signals at said first and second input terminals.

4. The combination comprising:

a first input terminal;

first and second transistors each having an emitter, a collector and a base;

means connecting the emitters of the first and second transistors together and by way of a resistor to said first input terminal;

third and fourth transistors having their collectors connected to said first input terminal and having their emitters connected to the bases of the first and second transistors, respectively;

a first capacitor connected between the base of the first transistor and a point-of-reference potential;

a second capacitor connected between the base of the second transistor and said point-of-reference potential; second and third input terminals coupled respectively to the bases of the third and fourth transistors;

means for applying bivalued input signals at the second and third input terminals;

means for applying a control signal at the first input terminal; and

means for connecting first and second output loads at the collectors of the first and second transistors, respectively.

5. The combination comprising:

a first input terminal;

first and second transistors of one conductivity type each having an emitter, a collector and a base;

means for connecting output loads at the collectors of the first and second transistors, respectively;

means connecting the emitters of the first and second transistors together and by way of a resistor to said first input terminal;

third and fourth transistors of said one conductivity type having their collectors connected to said first input terminal and having their emitters connected to the bases of the first and second transistors, respectively;

a first capacitor connected between the base of the first transistor and a point-of-reference potential;

a second capacitor connected between the base of the second transistor and said point-of-reference potential;

second and third input terminals coupled respectively to the bases of the third and fourth transistors;

means for applying bivalued input signals at the second and third input terminals; and

means for switching the voltage at the first input terminal between first and second values, the first value being incapable of rendering either one of the first and second transistors conducting, and the second value being sufficient in magnitude to bias one of the first and second transistors in the on condition and to forward bias the collector-base junction of the associated one of the third and fourth transistors.

6, The combination with a flip-flop having first and second input points and corresponding first and second output terminals, the combination of:

first and second transistors each having an emitter, a collector and a base;

means coupling the collectors of the first and second transistors to different ones of the first and second input points of the flip-flop;

an input terminal;

means connecting the emitters of the first and second transistors in common, and by way of a common emitterresistor to said input terminal;

third and fourth transistors having their collector electrodes coupled to said input terminal and having their emitters respectively connected to the bases of the first and second transistors;

a first capacitor connected between the base of the first transistor and a point-of-reference potential;

a second capacitor connected between the base of the second transistor and said point-of-reference potential; and

means for applying a control signal at said first input terminal.

7. The combination as claimed in claim 6, wherein the first, second, third and fourth transistors are of the same conductivity type, and including means coupling the first and second output terminals to the bases of different ones of said third and fourth transistors 8. The combination as claimed in claim 6, including a second flip-flop having first and second output terminals, and means for coupling the first and second output terminals of said second flip-flop to the bases of different ones of said third and fourth transistors.

9. A steering arrangement for a flip-flop having first and second input points, said steering arrangement comprising:

first and second transistors each having a base, an emitter,

and a collector electrode;

an input terminal;

a relatively constant impedance means connecting the emitter electrodes of the first and second transistors to said input terminal;

first capacitor means connected between the base electrode of the first transistor and a point-of-reference potential;

second capacitor means connected between the base electrode of the second transistor and said point-of-reference potential;

ll 12 first and second input circuit means coupled at the base rent to flow from said capacitance through the base-toelectrodes of the first and second transistors, respectively. emitter path of said transistor and through said relatively for selectively charging the first and second capacitor constant impedance means into said input terminal; said means, respectively, to a first charge condition; relatively constant impedance means limiting the current means for connecting the collector electrodes of the first le l by presenting to said capacitance an impedance and second transistors to the first and second input points l Value to the ohmic Value of Said relatively of said flip-flop, respectively; and stant impedance means multiplied by the current gain of means for applying a signal to said input terminal, said translstof relldeled f f I signal having a magnitude to render conductive the first The combmfmon as chimed i f 9 531d or second transistor when its associated capacitor means I0 relanvely constant Impedance means is a reslswn is charged to said first charge condition, for causing cur- 

1. The combination comprising: first and second transistors of one conductivity type each having an input electrode, an output electrode and a common electrode; means for connecting first and second output loads at the output electrodes of the first and second transistors, respectively; an input terminal; a relatively constant impedance means connected in common at one end to each one of said common electrodes and at the other end to said input terminal for increasing the impedance between the input electrodes of said transistors and said input terminal; first and second capacitors each being connected between the input electrode of a different one of said first and second transistors and a point-of-reference potential; third and fourth transistors connected to the input electrode of the first and second transistors, respectively, for selectively charging said first and second capacitors, respectively, to a first charge condition; and means for applying a control signal at said input terminal, said control signal having a magnitude to render conductive the first or second transistor when its associated capacitor is charged to said first charge conditiOn.
 2. The combination as claimed in claim 1 wherein said relatively constant impedance means is a resistor.
 3. The combination comprising: first and second transistors each having an input electrode, an output electrode and a common electrode; means for connecting first and second output loads at the output electrodes of the first and second transistors, respectively; first and second input terminals; a first resistor connected in circuit between the common electrode of the first transistor and the first input terminal; a second resistor connected in circuit between the common electrode of the second transistor and the second input terminal; a first capacitor connected between the input electrode of the first transistor and a point-of-reference potential; a second capacitor connected between the input electrode of the second transistor and said point-of-reference potential; first and second signal controlled input means connected to the input electrodes of the first and second transistors, respectively, to control the charge on the first and second capacitors, respectively; and means for applying control signals at said first and second input terminals.
 4. The combination comprising: a first input terminal; first and second transistors each having an emitter, a collector and a base; means connecting the emitters of the first and second transistors together and by way of a resistor to said first input terminal; third and fourth transistors having their collectors connected to said first input terminal and having their emitters connected to the bases of the first and second transistors, respectively; a first capacitor connected between the base of the first transistor and a point-of-reference potential; a second capacitor connected between the base of the second transistor and said point-of-reference potential; second and third input terminals coupled respectively to the bases of the third and fourth transistors; means for applying bivalued input signals at the second and third input terminals; means for applying a control signal at the first input terminal; and means for connecting first and second output loads at the collectors of the first and second transistors, respectively.
 5. The combination comprising: a first input terminal; first and second transistors of one conductivity type each having an emitter, a collector and a base; means for connecting output loads at the collectors of the first and second transistors, respectively; means connecting the emitters of the first and second transistors together and by way of a resistor to said first input terminal; third and fourth transistors of said one conductivity type having their collectors connected to said first input terminal and having their emitters connected to the bases of the first and second transistors, respectively; a first capacitor connected between the base of the first transistor and a point-of-reference potential; a second capacitor connected between the base of the second transistor and said point-of-reference potential; second and third input terminals coupled respectively to the bases of the third and fourth transistors; means for applying bivalued input signals at the second and third input terminals; and means for switching the voltage at the first input terminal between first and second values, the first value being incapable of rendering either one of the first and second transistors conducting, and the second value being sufficient in magnitude to bias one of the first and second transistors in the on condition and to forward bias the collector-base junction of the associated one of the third and fourth transistors.
 6. The combination with a flip-flop having first and second input points and corresponding first and second output terminals, the combination of: first and second transistors each having an emitter, a collector and a base; means coupling the collectors of the first and second transistors to different ones of the first and second input points of the flip-flop; an input terminal; means connecting the emitters of the first and second transistors in common, and by way of a common emitter-resistor to said input terminal; third and fourth transistors having their collector electrodes coupled to said input terminal and having their emitters respectively connected to the bases of the first and second transistors; a first capacitor connected between the base of the first transistor and a point-of-reference potential; a second capacitor connected between the base of the second transistor and said point-of-reference potential; and means for applying a control signal at said first input terminal.
 7. The combination as claimed in claim 6, wherein the first, second, third and fourth transistors are of the same conductivity type, and including means coupling the first and second output terminals to the bases of different ones of said third and fourth transistors.
 8. The combination as claimed in claim 6, including a second flip-flop having first and second output terminals, and means for coupling the first and second output terminals of said second flip-flop to the bases of different ones of said third and fourth transistors.
 9. A steering arrangement for a flip-flop having first and second input points, said steering arrangement comprising: first and second transistors each having a base, an emitter, and a collector electrode; an input terminal; a relatively constant impedance means connecting the emitter electrodes of the first and second transistors to said input terminal; first capacitor means connected between the base electrode of the first transistor and a point-of-reference potential; second capacitor means connected between the base electrode of the second transistor and said point-of-reference potential; first and second input circuit means coupled at the base electrodes of the first and second transistors, respectively, for selectively charging the first and second capacitor means, respectively, to a first charge condition; means for connecting the collector electrodes of the first and second transistors to the first and second input points of said flip-flop, respectively; and means for applying a signal to said input terminal, said signal having a magnitude to render conductive the first or second transistor when its associated capacitor means is charged to said first charge condition, for causing current to flow from said capacitance through the base-to-emitter path of said transistor and through said relatively constant impedance means into said input terminal; said relatively constant impedance means limiting the current level by presenting to said capacitance an impedance equal in value to the ohmic value of said relatively constant impedance means multiplied by the current gain of the transistor rendered conductive.
 10. The combination as claimed in claim 9 wherein said relatively constant impedance means is a resistor. 